THIS INVENTION relates to pulverisers and to a method of pulverising.
In many industries it is necessary to reduce pieces of material to fine powder. An example is coal which is reduced from nuggets to powder before being burned in certain types of power station furnace. Limestone, chalk and many other minerals must also, for most uses, be reduced to powder form.
Breaking up of the-rock and grinding it into powder has, to the best of Applicant""s knowledge, heretofore mainly been carried out mechanically. Ball mills, hammer mills and other mechanical structures which have moving parts that impact on, and hence crush, the pieces of material are widely used.
It has also been proposed that pieces of material should be broken up in a moving airstream. In prior U.S. Pat. No. 2,832,454 an airstream is blown at supersonic speed from a nozzle into a draft tube within which its speed falls to subsonic. Particles are sucked into the draft tube through an annular gap between the draft tube and the nozzle and broken up in the draft tube. In U.S. Pat. No. 5,765,766 pieces to be broken up fall into an airflow tube, are carried by the air flow into a disintegration chamber and blown against an anvil which breaks up the pieces. In both these structures the pieces are blown into the disintegration zone by air moving means upstream of the disintegration zone.
In U.S. Pat. No. 3,255,793 air is sucked by a centrifugal fan through a tube of circular and constant cross section. The tube is connected to the fan casing in which the fan rotor turns by a diverging conical nozzle. The U.S. specification states that the pieces entering the nozzle explode due to the fact that the air pressure in the nozzle is below the internal pressure of the particles.
The present invention seeks to provide a new pulveriser and a new method of pulverizing.
According to one aspect of the present invention there is provided a pulveriser which comprises an air flow pipe including a venturi, air moving means for inducing an air flow through said venturi at a speed of Mach 1 or faster, and an inlet to said pipe upstream of said venturi through which pieces of frangible material can be fed into said pipe, said air moving means having a suction inlet thereof connected to the outlet of said venturi.
Said air moving means can be a centrifugal fan having its suction inlet co-axial with a fan rotor thereof and its outlet tangential to the fan rotor.
Said venturi may comprise a throat, a convergent portion which decreases in area from an air inlet end thereof to said throat, and a divergent portion which increases in area from said throat to an air outlet end thereof.
Said portions are preferably both circular in cross section.
To prevent pieces of more than a predetermined size reaching said venturi, means for screening the material can be provided. The pulveriser can also comprise means for feeding said solid pieces of material as a stream of pieces which are spaced apart in the direction in which they are travelling.
Said means can be an inclined rotatable feed screw for lifting pieces which have passed through a screen which prevents pieces of greater than predetermined size reaching said screw, the pieces being discharged from the top end of the screw so that they drop into said pipe.
According to a further aspect of the present invention there is provided a method of pulverising frangible material in which air is sucked through a venturi at a speed equal to or in excess of Mach 1, and the pieces of material to be pulverised are entrained in the air flowing to the venturi so that they are carried to the venturi by the flowing air.
To achieve efficient operation without blocking, said pieces are preferably separated into a stream of pieces which reach said venturi in succession. Said material can additionally be screened to prevent material pieces above a predetermined size reaching said venturi.